In closed cryogenic refrigeration systems, high purity grades of a working gas (i.e., helium) are used. Cooler temperatures are produced when unwanted gases (i.e., nitrogen and oxygen) are eliminated from the cryogenic system. A technique called purging and charging aids in the process of obtaining the desired high purity grade of the working gas. In the purging and charging technique, working gas from a source is allowed to enter the cryogenic system in a controlled manner. That is, the cryogenic system is charged with the working gas. After a desired amount of working gas has entered the system the transferring of working gas from the source to the system is temporally ceased. The cryogenic system is then vented or purged of gas within the system which at this time is a mixture of the working gas and other gases. After a certain amount of the unwanted mixture of gases is vented from the system the purging is ceased. The cryogenic system is then recharged with working gas from the source and then repurged in an alternating manner until a raised level of concentration or high purity grade of the working gas exists within the system.
Various devices which enable this purging/charging process have been developed. In a typical purging/charging device, an assembly of tubes and valves is employed. A first tube connects the working gas source with the cryogenic system. A second tube is connected to the first tube and provides a relief port for the cryogenic system. A first valve is used to open one end of the first tube to the cryogenic system. A second valve is used to open the opposite end of the first tube connected to the source of working gas. A third or relief valve is used to open and close the second tube.
In order to charge the system with a supply of the working gas, the first two valves must be opened and the third valve closed. After the desired amount of working gas has been transferred to the cryogenic system, the first or system valve is closed to temporarily isolate the system, and the second or supply valve is closed to stop the supply of working gas from the source. The relief valve on the second tube is opened to provide an outlet for the system. The system valve is then opened to allow purging of the system. After sufficient purging, the system valve is closed to allow the relief valve to be closed and the supply valve to be reopened for recharging. Upon reopening of the system valve the cryogenic system is recharged, and so on.
Accordingly, the purging and charging process involves the opening and closing of the three valves which becomes cumbersome and is often confusing and awkward. A further disadvantage of such purging/charging devices is that one or more of the valves may not be completely closed causing unaccounted leakage, and hence the purging and charging is not accurately accomplished.